Commercial devices which effectively handle suspensions, such as paper pulp, at medium consistency, that is at about 6-15% solids consistency, are known. It is also known that air or, more generally gas, if present in the fiber suspensions causes problems in almost all process stages in the pulp and paper industry. When pulp is pumped, mixed, screened, washed or otherwise handled without excess gas significant savings in equipment, power consumption and the like can be achieved. For instance, one device which has been particularly successful in allowing handling of gas-containing medium consistency fiber suspensions is a fluidizing centrifugal pump which simultaneously pumps and degasses the suspension. Typically, such pumps utilize a separate vacuum pump, piping from the centrifugal pump to the vacuum pump, a separate motor and motor mount for the vacuum pump, etc., in order to exhaust the gas which has been separated from the suspension so that the suspension may be effectively pumped by the pump impeller.
U.S. Pat. No. 3,230,890 discloses a centrifugal pump for removing gas from low consistency suspensions or from water having either a built-in vacuum pump or an external vacuum pump.
A fluidizing centrifugal pump having a built-in vacuum pump is disclosed in U.S. Pat. No. 4,776,758. FIG. 1 illustrates the prior art centrifugal pump, with the volute being omitted, and provided with a vacuum pump on the same shaft as the impeller. The characteristic features of the prior art pumps on the market today and which have not, however, proven to be successful due to some shortcoming in the structure thereof, are disclosed in detail in the following. The prior art pump of FIG. 1 has a fluidizing impeller 12 rotating in an ordinary medium consistency pump housing. The impeller 12 has through bores or openings 14 for allowing the air accumulated at the front side of the impeller 12 to be drawn by means of the vacuum pump 10 toward the back side of the impeller 12. The impeller is also equipped with so-called back vanes 16 on the back side thereof for separating the fiber suspension from the medium being drawn through the openings 14 in the impeller plate 18. The main purpose of the back vanes 16 is to pump the fiber suspension back to the pump volute and thus prevent the fibers from entering the vacuum pump 10, as the risk of damaging the vacuum pump 10 rises dramatically if the fibers are allowed to enter the vacuum pump 10. The vacuum pump 10 is a so-called liquid ring pump which has been arranged on the pump shaft 20 behind an intermediate plate 22 in which only a narrow ring-shaped duct 24 is provided which duct surrounds the shaft 20 or the impeller extension 26 for allowing the gas to flow towards the vacuum pump. The intermediate plate 22 is also provided with a ring-shaped channel 28 and a narrow duct 30 leading thereto for introducing make-up air to the vacuum pump while the pump is running. The duct 30 is connected via channel 32 to a vacuum regulating valve (not shown). As the flow of make-up air is not continuous and the duct is very narrow, fibers tend to accumulate therein and lead to the clogging thereof. The vacuum pump housing 34 is provided with a conduit 36 for feeding liquid to the liquid ring pump 10 for maintaining the amount of liquid substantially constant therein. Conduit 36 is connected to the outer, eccentric circumference 38 of the liquid ring pump 10. In other words, the conduit 36 leads exclusively and directly to the liquid ring. The suction opening for the liquid ring pump 10 is provided, naturally, on the side of the centrifugal impeller 12. The discharge channel (not shown) for the gas to be removed from the pump 10 is arranged at the opposite side of the vacuum pump 10, i.e. on the back side of the vacuum pump relative to the centrifugal impeller 12.
Various problems have, however, been encountered with the pump in operation today. For example, the air removal capacity has been significantly lower than required, i.e. the vacuum created has not reached a sufficiently high level. Also, the discharge pressure of the vacuum pump has been found to be too low. In some cases, it is desirable to reintroduce the material discharged from the vacuum pump, a mixture containing mainly gas but also some fibers, back into the top portion of a mass tower to recover the fibers. If, however, the discharge pressure of the vacuum pump is too low the pumped material cannot be conveyed to the top of the mass tower, and an additional pump must be installed for that purpose. Also, the open annular volume in the intermediate plate 22 of prior art pump has a tendency to become clogged by the fibers. The prior art pump also lacks any means for introducing sealing liquid between the vacuum pump rotor and the vacuum pump chamber.
The axial gap 40 in the prior art pump between the vanes 42 of the vacuum pump 10 and the inner walls 44 of the vacuum pump housing is about 0.4 mm. The reason for such large clearance is the fact that there are a number of factors that make it impossible to decrease the clearance 40 as the various components of the pump are installed on the shaft 20 or around the shaft 20 starting from the drive end 46 of the shaft. Thus, the dimensions of the components also effect the clearance 40. The result of too wide a clearance is, of course, excess leakage and an insufficient vacuum. Another reason for the wide gap 40 may also be the fact that the shaft 20 of the pump tends to flex slightly during the operation of the pump creating the risk of mechanical contact between the vacuum pump vanes 42 and the housing walls 44. Thus, the large clearance 40 is necessitated by considerations ensuring a long-lasting operation of the pump.
The pump in accordance with the present invention is designed to solve the above problems. The pump of the present invention provides means for introducing a sealing liquid to the clearances between the vacuum pump rotor and adjacent side walls for sealing the same and thus increasing the pumping action of the device. Make-up air for controlling the vacuum of the pump and for maintaining the vacuum at a constant level may be provided at the back wall of the vacuum pump chamber thereby avoiding the narrow and curved or angled make-up air ducts of the prior art pumps as well as eliminating friction factors which led to a decreased flow of make-up air.
In addition, the centrifugal pump of the present invention with the built-in rotary vacuum pump avoids the narrow flow channels of the prior art which were subject to blocking by the fibers in case the fibers entered these air ducts.
The pump is also provided with means for introducing a flushing liquid into critical locations of the instant pump so as to avoid blocking thereof by the fiber suspension.
As is described in more detail below, the sealing liquid may be introduced separately to one or both sides of the vacuum pump chamber so that it can flow into and seal the space or clearance between the pump rotor and adjacent side walls of the vacuum pump. The sealing liquid may also be fed to the spaces through a single conduit leading through the central portion of the vacuum pump rotor.
A control valve for regulating the vacuum of the vacuum pump may also be directly attached at the end of the make-up air channel.
Means are also provided to introduce a liquid into the pump for replenishing the liquid ring which is partially exhausted with the air during rotation. Finally, the vacuum pump rotor central portion may be tapered toward the gas outlet ports so as to prevent the formation of a gas pocket around the rotor central portion.
The centrifugal pump impeller may be provided with a rotor having fluidizing blades either within the pump inlet or entirely outside the pump inlet or any combination thereof.
The pump is utilized for pumping gas containing media such as filtrates from industrial filtering devices and fiber suspensions in the pump and paper industry which may also contain substantial amounts of gas.